Cumene is an important intermediate in the chemical and polymer industries. The majority of all cumene manufactured in the world today is used for the production of phenol. The demand for phenol for the manufacture of Bisphenol-A and subsequently polycarbonates is accelerating, owing to the broadening applications of polycarbonates in the electronic, healthcare, and automobile industries.
Cumene is typically produced by alkylating benzene with a C3 alkylating agent, such as propylene, under liquid phase or mixed gas-liquid phase conditions in the presence of acid catalysts, particularly zeolite catalysts. In addition to cumene, the process produces diisopropylbenzene (DIPB), some triisopropylbenzene (TIPB) and other heavy by-products so it is conventional to transalkylate the polyisopropylbenzenes (PIPB) with benzene to generate additional cumene. The product of the transalkylation reaction is then fed, together with the alkylation reaction effluent, to one or more benzene columns, to recover unreacted benzene, then to one or more cumene columns, to recover the desired cumene product. The bottoms of the cumene column(s) is typically further distilled in one or more PIPB columns to recover most of the DIPB and part of the TIPB for recycle to the transalkylator. The remainder of the TIPB and essentially all of the compounds heavier than TIPB are typically discharged at the bottoms of the PIPB column as the residue. An aromatic purge is also generally taken at the overhead of PIPB column to remove excess butylbenzenes and cymenes produced in the alkylation reactor due to the presence of butene in the propylene feed and toluene in the benzene feed. This aromatic purge typically represents a yield loss of about 0.3 to 1.0%, even though the total concentration of butylbenzenes and cymenes in this aromatic purge is typically less than 10 weight %, the balance being mainly recoverable compounds such as cumene and DIPB.
It has now been found that, if at least part of the aromatic purge is recycled to the alkylation reactor, butylbenzene and cymene within the aromatic purge can be alkylated to useful cumene precursors and compounds heavier than triisopropylbenzene. The heavier compounds can be effectively removed from the system downstream of the alkylation reactor, whereas the cumene precursors can be recycled to produce additional cumene. Recoverable compounds contained within the aromatic purge, such as cumene and DIPB, are partly converted to recoverable cumene precursors in the alkylator, and fully recycled to produce additional cumene. Accordingly, an improvement in cumene process yield can be achieved.